Photosensitive electrodes and method of making same



May 3), 1967 M|K|o AsHnKAwA ETAL 3,322,570

PHOTOSENSITIVE ELECTRODES AND METHOD OF' MAKING SAME Filed Ma,r 2l, 1963F/GZ United States Patent O 3,322,570 PHOTOSENSITIVE ELECTRODES ANDMETHOD F MAKING SAME Mikio Ashikawa, Tokyo, Hideki Umezawa,Hachioji-shi, Tokyo, and Kazuo Ozawa, Tokyo, Japan, assignors toHitachi, Ltd., Tokyo, Japan, a corporation of Japan Filed May 21, 1963,Ser. No. 281,959 Claims priority, application Japan, May 25, 1962,37/20,917 7 Claims. (Cl. 117-211) This invention relates generally tophotoconductive layers and more particularly to photosensitiveelectrodes for Vidicon and other pickup tubes and methods of making suchelectrodes.

In the past, photosensitive electrodes of the kind described havegenerally included a composite layer including a lirst and porous layerof photoconductive maaterial vapor-deposited on the electricallyconductive substrate in the atmosphere of low pressure gas and a secondand solid, glassy layer of photoconductive material vapordeposited onthe first layer in vacuum or one comprised of three successivelysuperposed layers including a first and a second solid layer with aporous layer therebetween. Porous layers are characterized by theirextremely high electrical resistivity and fast photoresponse but areusually low in photosen-sitivity. In contrast, solid layers generallyhave a photosensitivity higher than that of porous layers but are slowin photoresponse and lower in electrical resistivity. Thus the use of acomposite layer comprised of two or three elementary layers as describedabove has been for the purpose of obtaining a photosensitive electrodehaving satisfactory characteristics as a whole by allowing .the porousand solid layers to have their dernerits offset by each other.

However, since the characteristics of such composite layer are largelyinfluenced by the magnitude `and proportion of the thicknesses of therespective elementary layers, the gas pressure under which the porouslayer is vapordeposited and other factors, it has been diiiicult toobtain a composite layer having desired stable and constantcharacteristics.

The present invention is intended to provide a photosenitive electrodewhich includes a single-layer apparently easier to form than anymultiple composite layer and has characteristics at least compatible tothose of any convention-al photosensitive electrode, which has amultiplelayer and to provide a method of making such photosensitiveelectrode having a single-layer.

According to the method of the present invention, the vapor depositionof a photoconductive material is performed in a gaseous atmosphere whilevarying the pressure thereof in a continuous fashion from a high to alow vacuum or vice verse to obtain a photosensitive electrode comprisedof a vapor-deposited photoconductive layer varying continuously inporosity through the thickness of the layer.

It will be appreciated that the electrode made according to the presentinvention includes a photoconductive layer which is apparently asingle-layer but has characteristic properties combining the merits of aporous and ya solid iilm. In addition, such characteristic propertiescan be controlled with ease by controlling the rate at which the gaspressure is varied.

The present invention will now be described in further detail by apractical example of the typical procedure in connection with theaccompanying drawings, in which:

FIGURE 1 illustrates a portion of a pick-up tube employing aphotosensitive electrode according to the present invention, and

FIGURE 2 illustrates acharacteristic performance of the photosensitiveelectrode according to the present invention.

In FIGURE 1, there is shown a tubular glass envelope 1 and 7at one endof this envelope 1 a photosensitive electrode 2 which according to thisinvention is hermetically sealed to envelope 1 through a metal ring 6.

The photosensitive electrode 2 also serves as a window for the envelope1 and comprises a transparent substrate 3 `such as a glass plate havinga transparent, electrically conductive layer 4, for example, a tin oxidelayer, coated on the inner surface of the substrate 3 and a, singlelayer 5 0f photoconductive material such as antimony trisuliide,vapor-deposited on said conductive layer 4.

Adjacent to and facing the inner surface of the photosensitive electrode2, but ata slight distance therefrom, is provided a metal mesh electrode7 carried =by a metal cylinder 8.

A practical example for making the photosensitive electrode according tothe present invention is as follows. In this example, it is .assumedthat antimony trisulfde is employed as a photoconductive material andargon as an ambient gas. A substrate made of glass plate carrying atransparent electrically conductive layer on one surface of said glassplate and antimony trisuliide being contained in a Crucible is rstplaced in a vacuum evaporation unit. Then the unit is evacuated and theCrucible is heated to evaporate -antimony trisulide. The timerequirement for the vapor deposition of antimony trisullide is measuredin advance, which is usually a few minutes. The opening of the gas valvein the argon gas inlet se-ction is positioned and the throttle in thegas inlet conduit is set in accordance with the time requirement. Thus,in case the deposition time -required is, for example, tive minutes, theargon gas inlet section is set up so that the pressure in theevaporation unit is varied from 104' mm. Hg to 10-l mm. Hg in tiveminutes. In this case, if the pressure in the evaporation unit is variedin proportion to the time elapsing after the vapor-depositing has beenstarted, the layer va-pordeposited on the substrate will have a porositylinearly varying through the thickness of the layer, ranging from anextremely compact solid structure obtained under the pressure of l0*5mm. Hg. to a porous structure obtained under the pressure of l0*1 mm.Hg.

One example of the characteristic performance of the photosensitiveelectrode 4is illustrated in FIGURE 2. The illustration represents theso-called light transfer characteristics of Vindicon tubes, whichclearly expresses the sensitivity of the tubes. In the drawing, theabscissa represents the intensity of illumination on the surface of thephotosensitive electrode while the ordinate represents the magnitude ofthe signal output current of the Vidicon, -both in a logarithmic scale.The signal output current has been determined for a predetermined darkcurrent of 0.02 na. for each Vidicon.

Also in FIGURE 2, lines 1, 2 and 3 illustrate the characteristics ofrespective Vidicons employing a photosensitive electrode of the presentinvention, one including a three-layer, layer, and one including atwo-layer, layer, respectively.

At present, `it is generally considered that the best way of cpmparingthe sensitivity between Vidicons is to determine the intensity ofillumination on the surface of the photosensitive electrode as requiredto allow the Vidicon to provide a signal output of a specific currentvalue. As will be observed in the chart, the values of the intensity ofillumination determined from lines 1, 2 and 3 for ya signal outputcurrent of 0.1 laa. are 0.35 lux, 2.8 lux and 18 lux, respectively. Inother words, the light requirement of the photosensitive electrode madeaccording to the present invention is about one-eight of tha-t of thethreelayer, layer electrode and about 1/,1 of that of the twolayer,layer electrode, that is, the inventive photosensitive electrode has ahighly improved sensitivity, which is about eight times and fty-onetimes as high as that of the threeand two-layer film electrodes,respectively.

Moreover, it will be apparent from the foregoing that the variation inporosity of the photoconductive layer thr-ough its thickness can beotherwise controlled by varying the gas pressure in the evaporation unitat an appropriate rate instead of varyin g the pressure in proportion totime.

Thus, it will be seen that in the practice of this invention aphotoconductive layer having a porosity varying through its thickness inany desired manner can be o-btained by controlling the variation inpressure of the gasous atmosphere within the evaporation unit.Accordingly, in the practice of the invention, the relationship betweenthe photosensitive electrode performance and the manufacturing methodcan be determined easily compared with the case of conventionalphotosensitive electrode having twoor threecomposite layers ofphotoconductive material. Thus it is possible to make photosensitiveelectrodes which are improved in performance over conventional ones.

What we claim is:

1. A photosensitive electrode comprising a transparent substrate havinga transparent, electrically conductive layer on one surface thereof anda single layer of photoconductive material disposed on said electricallyconductive layer, said photoconductive material having a porosity whichvaries continuously from the solid state to the porous 4state or fromthe porous state to the solid state through the thickness of thephotoconductive layer.

2. A photosensitive elec-trode according to claim 1, wherein theporosity varies continuously and linearly or randomly from the solidstate to the porous state or from the porous state to the solid statethrough the thickness of the photoconductive layer.

3. A photosensitive electrode according to claim 1,

wherein said photoconductive layer is .antimony trisultide.

4. A method of making a photoconductive electrode which comprisesplacing a transparent substrate having a transparent, electricallyconductive layer in an evaporation unit with a photoconductive material,and heating the photoconductive material to evaporate it while varyingthe pressure in said evaporation unit gradually from a rst predeterminedvalue to a second predetermined value which is higher or lower than saidfirst predetermined value, :to cause deposition of a single layer ofphotoconductive material on the surface of said electrically conductivelayer, said photoconductive material having a porosity varyingcontinuously through the thickness of the photoconductive layer.

S. The method of lclaim 4, wherein the porosity varies continuously andlinearly or randomly from a solid state to a porous state, or from aporous state to a solid state through the thickness of thephotoconductive layer.

6. The method of claim 4, wherein said photoconductive material isantimony `trisultide.

7. The method of claim 4, wherein the pressure is varied from about 10*5mm. Hg. to 10-1 mm. Hg.

References Cited UNITED STATES PATENTS 2,967,962 1/1961 Turk 117-106 X3,015,746 1/1962 Lubszynski 313-65 FOREIGN PATENTS 138,950 8/ 1960Russia.

ALFRED L. LEAVI'IT, Primary Examiner.

WILLIAM L. JARVIS, Examiner.

4. A METHOD OF MAKING A PHOTOCONDUCTIVE ELECTRODE WHICH COMPRISESPLACING A TRANSPARENT SUBSTRATE HAVING A TRANSPARENT, ELECTRICALLYCONDUCTIVE LAYER IN AN EVAPORATION UNIT WITH A PHOTOCONDUCTIVE MATERIALAND HEATING THE PHOTOCONDUCTIVE MATERIAL TO EVAPORATE IT WHILE VARYINGTHE PRESSURE IN SAID EVAPORATION UNIT GRADUALLY FROM A FIRSTPREDETERMINED VALUE TO A SECOND PREDETERMINED VALUE WHICH IS HIGHER ORLOWER THAN SAID FIRST PREDETERMINED VALUE, TO CAUSE DEPOSITION OF ASINGLE LAYER OF PHOTOCONDUCTIVE MATERIAL ON THE SURFACE OF SAIDELECTRICALLY CONDUCTIVE LAYER, SAID PHOTOCONDUCTIVE MATERIAL HAVING APOROSITY VARYING CONTINUOUSLY THROUGH THE THICKNESS OF THEPHOTOCONDUCTIVE LAYER.